Interpretive Summary: Apple scab is the most important fungal disease of apple worldwide. The disease is managed primarily through the timely application of fungicides. Numerous apple growers have lost or are in danger of losing several classes of very effective scab fungicides due to the development of fungicide resistant strains of the pathogen. Captan fungicide is one exception. It has been on the market for nearly 60 years and is still very effective against apple scab, but its use has been limited to specific timings during the growing season because it does not control the broad spectrum of apple diseases that some of the newer fungicides do. A field trial was designed to evaluate the utility of including Captan fungicide as a mixing partner with fungicides from classes prone to resistance development in a season long program for controlling apple scab. The trial was conducted in an orchard where resistant strains of the fungus exist for 2 of the 3 classes of fungicides being tested. We found that all treatments that included Captan fungicide as a mixing partner reduced apple scab significantly relative to treatments of the same fungicides applied without Captan. The results of this trial are valuable to apple growers because it shows that Captan can be uses as a reliable mixing partner for controlling apple scab and can prevent large-scale losses due to control failures attributed to fungicide resistance.

Technical Abstract:
Apple scab (Venturia inaequalis) is the most important fungal disease of apple worldwide. The disease is managed primarily through the timely application of fungicides. Numerous apple growers have lost or are in danger of losing several classes of very effective scab fungicides due to the development of fungicide resistant strains of the fungus, including dodine, the benzimidazoles and, most recently, the class of demethylation-inhibiting (DMI) fungicides. A primary measure for combating and/or managing fungicide resistance is to tank mix a contact fungicide with a fungicide at risk. A field trial was designed to evaluate the utility of including the contact fungicide Captan as a mixing partner with representative fungicides from classes prone to resistance development. Specifically, the strobilurin fungicide kresoxim methyl (Sovran), the EBDC fungicide mancozeb (Dithane), the benzimidazole fungicide thiophanate-methyl (Topsin-M), and the DMI fungicide myclobutanil (Nova) were applied singly or as mixing partner with the contact fungicide Captan in a season long schedule to manage foliar and fruit scab. The trial was conducted in a 'McIntosh' orchard known to harbor Venturia isolates resistant to the benzimidazole and DMI fungicides. With the exception of the Nova and Topsin-M treatments, all treatments significantly reduced fruit scab, and generally foliar scab, relative to the check treatment. Not surprisingly, the mixture of Captan with either Topsin-M or Nova significantly improved their control of both foliar and fruit scab. The use of Captan as a mixing partner with either Sovran or Dithane resulted in lower fruit scab than use of any of these products applied alone, although the reduction was not statistically significant. Results of this trial indicate that Captan is a reliable mixing partner for controlling scab when mixed with an ineffective fungicide.